To remove particulate matter from an exhaust gas from diesel engines, a particulate-matter-capturing ceramic honeycomb filter comprising a sintered ceramic honeycomb body with porous partition walls, through which the exhaust gas containing particulate matter passes, namely, a diesel particulate filter (DPF) has been developed and put into practical use. The ceramic honeycomb filter comprises a sintered ceramic honeycomb body having porous partition walls defining flow paths and a peripheral wall, and plugs alternately sealing both ends of the flow paths. Because the ceramic honeycomb filter is exposed to high temperatures, the sintered ceramic honeycomb body is made of heat-resistant cordierite ceramics having small thermal expansion coefficients, and plugging materials are the same cordierite ceramics as those of the honeycomb bodies such that there are small differences in thermal expansion between the plugs and the sintered ceramic honeycomb bodies.
When the exhaust gas containing particulate matter flows into such ceramic honeycomb filter, particulate matter in the exhaust gas are captured by fine pores in the porous partition walls. When the captured particulate matter is excessively accumulated in the ceramic honeycomb filter, there is likely to arise increased pressure loss in the filter, resulting in decrease in the engine power. Accordingly, the ceramic honeycomb filter is periodically regenerated by burning the captured particulate matter by an external ignition means such as an electric heater, a burner, etc. A pair of ceramic honeycomb filters are usually mounted onto an automobile, utilizing an alternate regeneration method, in which while one filter is regenerated, the other filter is used.
With respect to characteristics, the ceramic honeycomb filter is required not only to suffer low pressure loss to avoid the decrease of engine performance, but also to have enough thermal shock resistance to withstand thermal shock due to rapid temperature changes at regeneration, the stop of an engine, etc. Accordingly, various improvements have been proposed on plugs for the ceramic honeycomb filters.
JP63-28875B discloses a method for plugging open ends of a sintered ceramic honeycomb body, comprising plugging a sintered honeycomb structure by a cordierite-forming material batch, and then sintering it at a temperature of 1300° C. or higher to convert the cordierite-forming material batch to cordierite. This method provides a cordierite honeycomb filter having excellent thermal shock resistance and reliability, in which the predetermined open ends of the flow paths of the sintered ceramic honeycomb body are completely sealed.
JP2002-136817A discloses a ceramic honeycomb filter comprising sealing the predetermined open ends of flow paths of a sintered or unsintered ceramic honeycomb body with a plugging material comprising sintered powder and unsintered powder having the same composition as that of the sintered ceramic honeycomb body, and heating it at as high temperatures as 1400° C. to form plugs. Because the plugging material contains pulverized powder having the same composition as that of the sintered ceramic honeycomb body in this ceramic honeycomb filter, no cracking due to a thermal expansion difference occurs in the plugs or portions of the honeycomb structure near the plugs even at high temperatures, and there are no troubles such as the peeling of the plugs, etc.
It has been found, however, that when the plugging material is heated to a cordierite-forming temperature (for instance, 1300° C.) or higher to bond it to the sintered ceramic honeycomb body as in the above conventional technologies, it is difficult to make the thermal expansion coefficient of the cordierite ceramic honeycomb structure equal to that of the plugs. Namely, because plate-like kaolin particles in a cordierite material for the sintered ceramic honeycomb body are oriented when passing through a narrow slit of an extrusion die in the extrusion-molding of the material, cordierite crystals formed by sintering are also oriented, so that the resultant honeycomb structure has a small thermal expansion coefficient in a flow path direction and a radial direction. However, because the plugging material does not pass through the narrow slit of the extrusion die, cordierite crystals are randomly oriented, resulting in a relatively large thermal expansion coefficient. Accordingly, there is a large difference in a thermal expansion coefficient between the honeycomb structure and the plugs.
In addition, large residual stress is generated in interfaces between the plugs and the sintered ceramic honeycomb body at a bonding temperature of 1300° C. or higher. Large residual stress is likely to cause cracking in the plugs or in interfaces between the plugs and the honeycomb structure, and the peeling of the plugs, due to thermal shock by an exhaust gas and mechanical shock by engine vibration and vibration by contact with roads when mounted to automobiles.
JP63-24731B discloses a method for sealing predetermined flow paths of a porous ceramic honeycomb structure by forming openings in a film attached to the ends of the porous ceramic honeycomb structure at predetermined points, and charging a plugging material into the flow paths through the openings. In Example 3 of this reference, a slurry containing alumina cement and pulverized mullite is introduced into predetermined flow paths of a ceramic honeycomb structure under vibration, and the resultant plugs are cured at a temperature of 55° C. and a humidity of 90% for 2 hours, to integrate the plugs to the honeycomb structure. In this method, because the plug-bonding temperature is as low as 55° C., there is small residual stress in interfaces between the plugs and the ceramic honeycomb structure.
It has been found, however, that because the cordierite honeycomb structure has a small thermal expansion coefficient while the plugs composed of mullite and alumina cement have a relatively large thermal expansion coefficient, it is likely that cracking occurs between the ceramic honeycomb structure and the plugs by thermal shock by an exhaust gas, and that the plugs peel off, when mounted to an automobile.